1. Field of the Invention
The present invention relates to a polarization independent optical isolator and particularly to a bi-directional polarization independent isolator simultaneously and independently providing isolation to separate light-wave data channels propagating in opposite directions through an optical fiber.
2. Description of the Related Art
The use of optical fiber in long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity in the transmission of voice and/or data continues to increase, there is a continuing need to augment the amount of actual fiber-optic cable being used as well as to utilize the bandwidth of existing fiber-optic cable more efficiently. The latter practice of increasing the carrying capacity of existing fiber cable is sometimes referred to as the creation of xe2x80x9cvirtual fiberxe2x80x9d and is clearly more cost effective than adding real fiber.
One of the ways in which xe2x80x9cvirtualxe2x80x9d fiber is created is through the practice of Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or xe2x80x9cband.xe2x80x9d To increase data carrying capacity in a given direction, the number of such channels or bands must be increased.
Additionally, it is desirable to use existing fiber for bi-directional communications. Through the use of WDM, a single optical fiber may be used to transmit, both simultaneously and independently, both eastbound (northbound) as well as westbound (southbound) data. This bi-directional data-carrying capability of optical fiber further increases the need for additional channels. However, since all of the channels (wavelength bands) must reside within specific low-loss wavelength regions determined by the properties of existing optical fiber, increased channel capacity requires increased channel density. Thus, as the need for increased data carrying capacity escalates, the demands on WDM optical componentsxe2x80x94to transmit increasing numbers of more closely spaced channels with no interference or xe2x80x9ccrosstalkxe2x80x9d between them and over long distancesxe2x80x94becomes more severe.
Optical amplifiers are important components of fiber-optic communication systems. Traditionally, signal regeneration has been accomplished through the use of repeaters, which are combinations of demultiplexers, receivers, signal recovery electronics, transmitters (light sources together with optical modulators), and multiplexers. In a repeater, the signal for each channel is recovered electronically and transmitted anew. Unfortunately, the complexity and cost of repeater-based systems becomes unwieldy with the increase in the number of channels of WDM systems.
Optical amplifier systems have therefore become attractive alternatives to repeaters. Erbium-doped fiber amplifier (EDFA) systems have become especially popular owing to their gain characteristics near the 1.5 xcexcm transmission band.
Because of the indiscriminate and non-directional nature of optical fluorescence amplification, unless special precautions are taken, all signals will be amplified on transit through an EDFA and re-transmitted in both directions. These signals may include spurious signals caused by stray reflections or light scattering off of various optical components and propagating counter to the desired signal transmission direction.
To guard against amplification and subsequent transmission of such unwanted signals, optical amplifier systems generally include optical isolators on both sides of the optical gain element (the Er-doped fiber). As shown in the amplifier 100 of the prior art in FIG. 1, optical isolators, such as isolator 101 and isolator 102, are disposed to either side of an Er-doped fiber 103, and comprise part of a set of so-called xe2x80x9coptical passive componentsxe2x80x9d which are generally associated with optical amplifier systems. Other such optical passive components illustrated in FIG. 1 are Wavelength Division Multiplexers (WDM""s) 104 and 105 and bandpass filter 106. Also included in the amplifier 100 of the prior art shown in FIG. 1 are Co-Pump Laser 108 of 980 nm or 1480 nm and Counter-Pump Laser 110 of 980 nm or 1480 nm.
Optical isolators act as xe2x80x9cone-way gatesxe2x80x9d which only permit signal transmission in the desired direction. This property, although essential, creates a problem for communications systems in which signals are carried in both directions within individual optical fibers, viz. the isolators would block one set of signals.
Therefore, in the current state of the art, separate amplifiers are used for eastbound (northbound) and westbound (southbound) communications channels as shown in the band bi-directional amplifier 200 of the prior art of FIG. 2. In the band bi-directional amplifier 200, the counter-propagating signals are respectively separated and re-combined on either side of the pair of optical amplifiers 206 and 207.
For instance, in FIG. 2, if the xe2x80x9cbluexe2x80x9d or relatively short wavelength band 201 shown as solid lines represents westward propagating signals and the xe2x80x9credxe2x80x9d or relatively long wavelength band 202 shown as dash-dot lines represents eastward propagating signals, then these two signals are separated and recombined by WDMs or circulators 203A and 203B. Between the two WDMs or circulators 203A and 203B, the blue and red signals propagate on separate physical optical fiber sub-paths 204 and 205, respectively, but to either side of each WDM, the westbound blue and eastbound red signals co-propagate along the same physical fiber pathways 211 and 212. Each of the fiber sub-paths 204 and 205 contains its own amplifier system, 206 and 207, respectively. Optional second amplifiers 208 and 209 may be placed in each of the fiber sub-paths and the locations between each of the resulting sequential amplifiers 206 and 208 or 207 and 209 corresponds to multi-access ports 210A and 210B in the blue and red sub-paths, respectively. Generally, each of the optical amplifier systems, 206 and 207 and, optionally, 208 and 209, shown in FIG. 2, comprises all of the optical passive and active components illustrated in FIG. 1 and possibly others. In particular, the amplifier 206 (and optionally 208) contains optical isolators that only permit westbound light propagation and the amplifier 207 (and optionally 209) contains optical isolators that only permit eastbound light propagation.
One example of the possible wavelength constitution of co-propagating bi-directional signals is illustrated in FIG. 3, showing the relative positions between light traveling in a xe2x80x9credxe2x80x9d band and light traveling in a xe2x80x9cbluexe2x80x9d band through a band bi-directional polarization independent isolator. For the example shown in FIG. 3, the terms xe2x80x9credxe2x80x9d band and xe2x80x9cbluexe2x80x9d band are meant as relative terms referring to light of a relatively longer wavelength (the xe2x80x9credxe2x80x9d band) and light of a relatively shorter wavelength (the xe2x80x9cbluexe2x80x9d band) and may not correspond to actual colors of red or blue produced by that light.
Referring now to FIG. 3, as an example, the xe2x80x9cbluexe2x80x9d band 301 and the xe2x80x9credxe2x80x9d band 302 occupy separate wavelength regions each wholly contained within the well-known fiber transmission band 303 centered near a wavelength of 1.55 xcexcm. For instance band 301 might represent the wavelength constitution of the westbound signal channel(s) 201 of FIG. 2 while band 302 might represent the wavelength constitution of the eastbound signal channel(s) 202. This type of bi-directional lightwave transmission scheme is termed xe2x80x9cband bi-directionalxe2x80x9d transmission herein. Other types of band bi-directional transmission schemes are possible. For instance, the xe2x80x9cbluexe2x80x9d band might correspond to all or a portion of the 1.3 xcexcm fiber transmission band while the xe2x80x9credxe2x80x9d band might correspond to all or a portion of the 1.55 xcexcm transmission band, etc.
Generally, there can be more than one channel per band, in that a channel is one particular signal, one particular conversation, or one particular computer sending data. A band is a collection of channels and is one wavelength range, and could be one channel or a collection of channels. Ideally, a channel travels in one wavelength. Because of its relatively low loss per meter, the 1.55 micrometer band of light is suitable for relatively longer haul telecommunications. The 1.3 micrometer band of light is suitable for relatively short haul telecommunications (20 kilometers, 30 kilometers, etc.).
Optical amplifiers are costly and complex components of optical data and telecommunications systems. The prior-art bi-directional optical amplification system shown in FIG. 2 uses two such amplifiers, effectively doubling the cost, complexity, and bulk relative to unidirectional transmission systems. This doubling of systems is necessitated by optical isolators, which are integral passive components of optical amplifiers, generally performing isolation in a unidirectional sense, regardless of the wavelength of light propagated through them.
Because nothing in the operation of prior-art polarization independent optical isolators as described with respect to FIGS. 1-3 changes its fundamental character with changing wavelength, such isolators generally perform their xe2x80x9cone-way gatexe2x80x9d function regardless of the wavelength of light which is input to them. Thus, in order to realize the function of a bi-directional optical amplifier as discussed above, the eastbound and westbound signals must be bifurcated and two optical amplifiers must be used as in FIG. 2, with each of the two optical amplifiers associated with its own set of unidirectional optical isolators as in FIG. 1. Clearly, the development of a bi-directional optical isolator, having the property that the direction in which isolation occurs depends upon wavelength, would obviate the need for two amplifiers in bi-directional photonic systems and would facilitate the development of a bi-directional amplifier. Such a development would have the advantage of reduced cost, bulk and complexity as compared with existing bi-directional optical amplification systems and would have the additional advantage of facilitating the incorporation of bi-directional amplification into existing fiber-optic cable.
Accordingly, it is an object of the present invention to solve the above-mentioned problems of the related art and to create such advantages, as described above.
Another object of the present invention is to provide a bi-directional optical isolator suitable for use in bi-directional amplifiers.
A further object of the present invention is to provide a band bi-directional polarization-independent optical isolator.
Also, an object of the present invention is to provide an interleaved bi-directional polarization-independent optical isolator.
Still a further object of the present invention is to provide a double-stage band bi-directional polarization-independent optical isolator.
An additional object of the present invention is to provide a double-stage interleaved bi-directional polarization-independent optical isolator.
To accomplish the above-mentioned objects, the present invention is an apparatus comprising a bi-directional polarization-independent isolator transmitting therethrough input light including first wavelengths received from a first fiber to a second fiber while preventing transmission therethrough of input light of the first wavelengths received from the second fiber to the first fiber. The apparatus of the present invention transmits therethrough input light of second wavelengths different than the first wavelengths and received from the second fiber to the first fiber while preventing transmission therethrough of input light of the second wavelengths received from the first fiber to the second fiber. The bi-directional polarization-independent isolator of the present invention divides the input light into components, and recombines the components into output light transmitted to one of the first fiber and the second fiber and based upon whether the components are changed in character by the bi-directional polarization-independent isolator.
Also to accomplish the foregoing objects, the present invention is a band bi-directional polarization-independent optical isolator receiving from a first fiber input light having first wavelengths and input light having second wavelengths different than the first wavelengths, and receiving from a second fiber input light having the first wavelengths and input light having the second wavelengths. The band bi-directional polarization-independent optical isolator of the present invention comprises a bi-directional polarization independent optical element, a reflector comprising a mirror/waveplate assembly, and a lens. The bi-directional polarization independent element divides the input light into components and selectively recombines the components into output light transmitted to one of the first fiber and the second fiber and based upon the plane of polarization of the components and if a change in character of the components has occurred during transmission through the band bi-directional polarization-independent optical isolator. The mirror/waveplate assembly reflects therefrom the components of the input light incident on the reflector after selectively changing the plane of polarization of the components of the input light incident thereon based upon the wavelength of the components of the input light incident thereon. The lens is positioned equidistantly between the reflector and the bi-directional polarization independent optical element and collimates the components of the input light onto the reflector and focuses the reflected components of the input light onto the bi-directional polarization independent optical element.
In addition, to accomplish the foregoing objects, the present invention is an interleaved bi-directional polarization-independent optical isolator receiving from a first fiber input light having first wavelengths and input light having second wavelengths different than the first wavelengths, and receiving from a second fiber input light having the first wavelengths and input light having the second wavelengths. The interleaved bi-directional polarization-independent optical isolator of the present invention comprises a bi-directional polarization independent optical element, a reflector comprising a non-linear interferometer, and a lens. The bi-directional polarization independent optical element divides the input light into components and selectively recombines the components into output light transmitted to one of the first fiber and the second fiber and based upon the plane of polarization of the components and if a change in character of the components has occurred during transmission through the interleaved bi-directional polarization-independent optical isolator. The non-linear interferometer reflects therefrom the components of the input light incident on the reflector after selectively changing the plane of polarization of the components of the input light incident thereon based upon the wavelength of the components of the input light incident thereon. The lens is positioned equidistantly between the reflector and the bi-directional polarization independent optical element, and collimates the components of the input light onto the reflector and focuses the reflected components of the input light onto the bi-directional polarization independent optical element.
In addition, the present invention is a twin band bi-directional polarization-independent optical isolator comprising a four-fiber ferrule and a bi-directional polarization independent optical element, a mirror/waveplate assembly, and a lens.
Further, the present invention is a twin interleaved bi-directional polarization-independent optical isolator comprising a four-fiber ferrule and a bi-directional polarization independent optical element, a non-linear interferometer, and a lens.
In both the twin band bi-directional polarization-independent optical isolator of the present invention and the twin interleaved bi-directional polarization-independent optical isolator of the present invention, the four-fiber ferrule includes a first fiber, a second fiber, a third fiber, and a fourth fiber. The first fiber receives input light having first wavelengths and outputs output light having second wavelengths different than the first wavelengths and output light having the first wavelengths. The second fiber corresponds to the first fiber and receives input light having third wavelengths and outputs output light having fourth wavelengths different than the third wavelengths and output light having the third wavelengths. The third fiber receives input light having the second wavelengths and outputs output light having the first wavelengths and output light having the second wavelengths. The fourth fiber corresponds to the third fiber and receives input light having the fourth wavelengths and outputs output light having the third wavelengths and output light having the fourth wavelengths.
Also, the present invention is a double-stage band bi-directional polarization-independent optical isolator in which input light travels through a band bi-directional polarization-independent optical isolator twice. The double-stage band bi-directional optical isolator includes a four-fiber ferrule including two input fibers and two output fibers. One of the output fibers is coupled to one of the input fibers through a polarization-preserving optical coupling.
Further, the present invention is a double-stage interleaved bi-directional polarization-independent optical isolator in which input light travels through an interleaved bi-directional polarization-independent optical isolator twice. The double-stage interleaved bi-directional optical isolator includes a four-fiber ferrule including two input fibers and two output fibers. One of the output fibers is coupled to one of the input fibers through a polarization-preserving optical coupling.
In addition, the present invention is a method of isolating input light having first wavelengths traveling from a first fiber to a second fiber from input light having the first wavelengths traveling from the second fiber to the first fiber, and isolating input light having second wavelengths traveling from the second fiber to the first fiber from input light having the second wavelengths traveling from the first fiber to the second fiber. The method of the present invention comprises transmitting through a bi-directional polarization-independent isolator the input light including first wavelengths received from the first fiber to the second fiber while preventing transmission therethrough of the input light of the first wavelengths received from the second fiber to the first fiber. The method of the present invention also comprises transmitting therethrough the input light of second wavelengths received from the second fiber to the first fiber while preventing transmission therethrough of the input light of the second wavelengths received from the first fiber to the second fiber, said bi-directional polarization-independent isolator dividing the input light into components, and recombining the components into output light based upon whether the components are changed in character by the bi-directional polarization-independent isolator.
These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.